Batrachotoxin-A-20-alpha-p-3H-benzoate (BTX-B) is a potent analog of batrachotoxin which exhibits a selective action on voltage-sensitive sodium channels in a variety of excitable tissues resulting in a decrease in resting membrane potential. Our study proposes to characterize the binding of specifically labeled 3H-BTX-B to its sodium channel site with respect to a number of parameters, including interactions with other sodium channel effectors, stoichiometry, pH and membrane potential. The primary experimental protocol involves direct binding measurements of labeled toxin to sodium channels in mouse cortex homogenates using a pellet assay procedure. These studies will be extended to other excitable tissues as well as more purified membrane preparations. The results of these studies, when analyzed in conjunction with existing electrophysiological and biochemical data on the effects of the toxin, should lead to a better understanding of voltage-sensitive sodium channels and provide the foundation for subsequent use of BTX-B as a molecular tool in the study of excitability mechanisms.